In recent years, there have been proposed various electric appliances which detect movement of a human body and perform an efficient operation for the purpose of energy conservation. For example, such an electrical appliance includes an infrared detection device employing a pyroelectric element as a detector for infrared. A normal infrared detection device focuses infrared from a detection area onto the pyroelectric element by use of a lens, for example. A current signal outputted from the pyroelectric element varies in response to a change in an amount of infrared received by the pyroelectric element.
For example, there has been known an infrared detection device 1P as shown in FIG. 17. The infrared detection device 1P includes a pyroelectric element 2, an I/V conversion unit 300, and a voltage amplification unit 400. The I/V conversion unit 300 converts a current signal outputted from the pyroelectric element 2 to a corresponding voltage signal. The voltage amplification unit 400 amplifies an output from the I/V conversion unit 300. In the infrared detection device 1P, the current signal outputted from the pyroelectric element 2 is converted to the corresponding voltage signal by the I/V conversion unit 300 and subsequently is amplified by the voltage amplification unit 400 and thereafter is inputted into a following detection circuit (not shown). Besides, in the configuration illustrated in FIG. 17, the voltage amplification unit 400 functions as a bandpass filter having a passing band which is equivalent to a frequency band (e.g., 0.1 Hz to 10 Hz) of a current signal generated in response to human body detection.
For example, a change in an environment temperature is likely to cause an undesired low-frequency component unrelated to a detection target (e.g., a human body) in the current signal outputted from the pyroelectric element 2. In view of this, there has been proposed the infrared detection device 1P which further includes a DC feedback circuit 200 connected between an output terminal and an input terminal of the I/V conversion unit 300 (see document 1: JP 3472906 B2, paragraphs 0037 to 0044, FIG. 9).
In the infrared detection device 1P disclosed in above document 1, the I/V conversion unit 300 includes a first operational amplifier 31 having an inverting input terminal connected to the pyroelectric element 2. Connected between an output terminal and the inverting input terminal of the first operational amplifier 31 is a capacitor C1. The capacitor C1 serves as a capacitive element for forming an AC feedback circuit. The first operational amplifier 31 has a non-inverting input terminal connected to a reference voltage source 202 configured to generate a reference voltage.
Further, the DC feedback circuit 200 is an integration circuit in which an operation amplifier 201 is connected to a capacitor C200 and a resistor R200. The operational amplifier 201 has a non-inverting input terminal connected to the output terminal of the first operational amplifier 31. The capacitor C200 is connected between an output terminal and an inverting input terminal of the operational amplifier 201. The operational amplifier 201 has the inverting input terminal connected via the resistor R200 to the reference voltage source 202 configured to generate the reference voltage. The operational amplifier 201 has the output terminal connected to the inverting input terminal of the first operational amplifier 31 via an input resistor R201.
The infrared detection device 1P which has the configuration explained in the above sends the undesired low-frequency component to the input resistor R201 in accordance with the output from the operational amplifier 201. Therefore, it is possible to suppress an effect, due to the undesired low-frequency component, on the voltage signal outputted from the I/V conversion unit 300.
However, in the infrared detection device 1P having the aforementioned configuration, since the input resistor R201 is connected to the input terminal of the I/V conversion unit 300, a noise component which occurs at the input resistor R201 may be inputted into the I/V conversion unit 300, and thus an S/N ratio of the I/V conversion unit 300 is likely to be reduced. Especially, to reduce a cut-off frequency (e.g., less than 0.1 Hz) and to suppress a thermal noise of the input resistor R201, the input resistor R201 needs to have a relatively high resistance, for example, a resistance in the range of TΩ (tera-ohms).
In view of downsizing of the infrared detection device 1P, the input resistor R201 is normally constituted by a resistive element incorporated into an IC (integrated circuit). The resistance of such a resistive element greatly depends on the temperature. Therefore, a variation in the resistance caused by a change in the temperature is increased with an increase in the resistance of such a resistive element. A decrease in the resistance of the input resistor R201 may cause an increase in a thermal noise of the input resistor R201. Thus, the S/N ratio of the I/V conversion unit 300 may be reduced.